1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display employing an antiferroelectric liquid crystal material.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2-173724 of Nippon Denso and Showa Shell Petroleum discloses a liquid crystal display employing an antiferroelectric liquid crystal material. This display realizes a wide view angle, a high-speed response, and a good multiplex property, and therefore, has been energetically studied.
FIG. 3 shows a cell of the liquid crystal display employing the antiferroelectric liquid crystal material. Polarizer plates 21a and 21b are arranged in a cross-Nicol relationship. The liquid crystal cell 22 is placed between the polarizer plates 21a and 21b so that the direction of the major axis of an average molecule of the liquid crystal material is in parallel with the polarization axis of one of the polarizer plates 21a and 21b when no electric field is applied. The cell 22 is black when no voltage is applied, and is white when a voltage is applied. FIG. 4(A) shows a hysteresis loop indicating changes in the light transmittance of the cell 22 and voltages applied to the cell 22. When the voltage applied to the cell 22 is increased to a value V1, the light transmittance starts to change, and when the voltage reaches a value V2, the light transmittance is saturated. When the voltage is decreased to a value V5, the light transmittance starts to decrease. When the voltage applied to the cell 22 has an opposite polarity and when its absolute value is increased to a value V3, the light transmittance starts to change, and when the voltage reaches a value V4, the light transmittance is saturated. When the value of the voltage is decreased to a value V6, the light transmittance starts to change. The cell 22 takes a second stable state (a ferroelectric state) if the voltage of a pulse applied thereto is higher than a specific value intrinsic to the antiferroelectric liquid crystal material and if the product of the width and height of the pulse is above a threshold. Under the same conditions but with an opposite polarity, the cell 22 takes a third stable state (a ferroelectric state). The cell 22 takes a first stable state (an antiferroelectric state) if the absolute value of the product of the width and height of the pulse is below the threshold.
When the voltage applied to the liquid crystal cell 22 is decreased before the light transmittance is saturated, the light transmittance decreases along the same hysteresis loop as that along which it has increased, as shown in FIG. 4(B).
FIG. 5 shows a matrix of electrodes for driving the antiferroelectric liquid crystal material. The electrodes include scan electrodes Y1 to Y128 and signal electrodes X1 to X160. A select voltage is applied successively to the scan electrodes Y1 to Y128. In synchronization with the select voltage, information signals are simultaneously applied to the signal electrodes X1 to X160. As a result, selected liquid crystal pixels are switched to display information. This is a time-division driving technique.
FIGS. 8(A) and 8(B) show a method of driving the antiferroelectric liquid crystal display, according to a prior art. A frame for driving the display consists of two scan periods, and each of the scan periods consists of a selected period and an unselected period. Each scan period involves a scan voltage, a signal voltage, and a synthesized voltage corresponding to the difference between the scan and signal voltages. The waveforms of two scan periods in each frame are symmetrical to each other with respect to 0 V, to prevent the liquid crystal material from burning and deteriorating. Each selected period involves two phases. A pulse width is 100 .mu.s. Each unselected period is about 25 ms. In each selected period, a scan voltage has a predetermined height and a signal voltage determines the height of a synthesized waveform to select one of the first to third stable states. The selected state is maintained during the following unselected period. Namely, a light transmittance determined in a given selected period is maintained during the following unselected period, to display required data.
This conventional method only provides a large light transmittance for displaying white and a small light transmittance for displaying black, and is incapable of realizing intermediate levels of light transmittance. Namely, this method hardly displays gradations. One conventional technique for displaying gradations is an areal gradation technique. This technique groups pixels and handles each group as a pixel. This technique requires a complicated drive controller, provides poor resolution, and realizes only a limited number of gradations. Japanese Unexamined Patent Publication No. 4-34417 discloses a ferroelectric liquid crystal display that modulates pulse widths to realize gradations. This disclosure drives a ferroelectric liquid crystal material, which has SmC phases and only one stable state. Accordingly, this disclosure is quite different from the present invention, which drives an antiferroelectric liquid crystal material which has SmCA * phases and employs a different panel structure and different driving waveforms.
FIGS. 6 and 7 show a time-division technique of driving a liquid crystal display, disclosed in Japanese Unexamined Patent Publication No. 2-173724. This technique writes a screen in two frames. The waveforms of voltages applied in the first and second frames are symmetrical to each other with respect to 0 V. FIG. 6 shows the waveforms of voltages for setting an ON state of the display and corresponding light transmittance of the display. FIG. 7 shows the waveforms of voltages for setting an OFF state of the display and corresponding light transmittance of the display. As shown in FIG. 6, a signal applied to the scan electrodes consists of three phases. The first phase resets the liquid crystal material to the OFF state (antiferroelectric state). The second phase maintains the state set by the first phase. The third phase determines whether or not the liquid crystal material must be put in the ON state (ferroelectric state). In FIG. 6, the third phase is above a threshold for setting the ferroelectric state, so that the liquid crystal material is set to the ON state (ferroelectric state). In FIG. 7, the third phase is below the threshold, so that the liquid crystal material maintains the OFF state (antiferroelectric state).
In this way, the prior art realizes only the three stable states for an antiferroelectric liquid crystal material, to display only black and white. The prior art is incapable of displaying gradations.